Digital electronic systems, and in particular personal computers, are undergoing significant reductions in size and increases in functionality with successive design iterations. Associated decreases in the dimensions of integrated circuit components and component input/output terminals have had a direct effect on the patterns of printed circuit board contacts.
Conventional techniques for solder bonding integrated circuit device packages to printed circuit boards are either approaching their technology limits or proving to be expensive to implement with the small dimensions typical of advanced designs. The prevailing technology, stencil printing of solder paste on printed circuit boards, is at its limits while the prevailing alternative technique, forming solder deposits on printed circuit boards by plating, is relatively expensive.
An analogous situation applies to the formation of solder deposit arrays on integrated circuit die, such as with flip-chip devices, or on ceramic integrated circuit packages. The reflow connection of solder balls or solder columns onto integrated circuit die or ceramic packages is likewise a complex and expensive endeavor.
Thus, there has developed a surge of interest in dispensing or jetting molten solder directly onto integrated circuit die (individually or in wafer form), ceramic integrated circuit packages, and printed circuit boards. There are numerous advantages to the use of a solder droplet dispensing or jetting technology when the quality of the molten solder droplets can be controlled in composition, volume, and deposit location. The ability to accurately locate the molten solder droplets as deposited on a substrate has proved to be a manageable task. However, the realization of a molten solder dispensing system which is reliable enough for a manufacturing application, to the extent that the molten solder droplets are consistent in composition, volume, and formation has yet to be realized.
U.S. Pat. No. 5,377,961 describes an electrodynamic pump for dispensing molten solder in droplets of relatively controlled volume. The electrodynamic pump described in this patent is a significant improvement over droplet dispensing machines utilizing electro-strictive transducers, such as piezoelectric devices, and as such defines the basics of a relatively controllable molten solder dispensing system. Though early tests established the viability of the electrodynamic pump for dispensing molten solder droplets, evaluation in a manufacturing setting, where repeatability of droplet dispensing, consistency of droplet composition, and stability of droplet size are crucial, indicated that refinements were needed. In this regard, testing determined that the molten solder located in close proximity to the nozzle aperture did not remain a homogeneous mixture of pure solder with time. Metallurgical analysis of solder ejection nozzle residues and camera analyses of droplet dispensing confirmed that there was a mixture of pure solder and oxidized tin and lead within the droplets, the combination resulting in the creation of droplets with non-uniform physical properties such as surface tension, melting point and affinity for other materials (wettability). Enclosing the nozzle area within a nitrogen gas (oxygen-free) environment did not solve the problems.
Testing of the electrodynamic solder pump determined that the solder droplets were dramatically different after a few hours of operation, notwithstanding the consistency of the input parameters which define the pressure pulses creating the single shot droplets. Further investigation confirmed that the instability was attributable to the nonhomogeneous nature of the solder near the nozzle aperture, the nonhomogeneous solder being composed of the earlier noted mix of pure solder with oxidized tin and lead.
The problem identified during the investigation of droplet uniformity over extended electrodynamic solder pump operation became one of determining what conditions around the nozzle, other than a nitrogen environment, would lead to uniform and stable dispensing of solder droplets over extended periods of time in the context of molten solder having both tin and lead. Solder alloys including other oxidizing metals are inherently so encompassed.